Cable, satellite and terrestrial networks are three major mediums to deliver digital broadcasting services to end customers. Unlike satellite and terrestrial transmission, cable channels do not exhibit significant time and frequency selectivity. Consequently, spectrally efficient modulations (i.e., 256-QAM and 1024-QAM), are employed in cable networks to meet the capacity demand of bandwidth-consuming services such as HDTV and VoD, and to boost the penetration of digital video broadcasting. Recently, low-density parity-check (LDPC) codes have been introduced in DVB-S2 and DVB-T2 standards because of their design flexibility, decoding simplicity and the universally excellent error correction performance over various channel types.
Out of the consideration for implementation simplicity and components inter-operability, the LDPC codes specified in DVB-S2 standards are strongly recommended to be reused for next generation DVB-C system. Nevertheless, it is well known that a LDPC code ensemble, optimized in the context of binary modulation, does not necessarily work well for higher-order modulations, which is due to the unequal error protections incurred by modulations. The asymptotic performance of multilevel coding (MLC) for infinite code length has been investigated and has proven its optimality as a capacity approaching strategy when multistage decoding (MSD) is employed. However, the MSD algorithm requires decisions from lower decoding stages to be passed on to higher stages, which results in large decoding latency that may be unacceptable to high-speed applications.
As is appreciated by those of skill in the art of communication systems, interleaving is a procedure for rearranging the order of a sequence to fulfill different objectives. For channels subject to selective fading over time and frequency domains, bit and/or symbol interleaving have been used in conjunction with channel coding to distribute the error bursts. In addition, bit interleaving is employed by concatenated codes, particularly Turbo codes, to scramble the information bits to the second constituent encoder so that a long random code can be generated.
As a result of LDPC codes, frameworks such as, for example, density evolution, differential evolution and extrinsic information transfer (EXIT) charts, have been invoked to design and analyze the degree profile of a code ensemble. In terms of the threshold SNR for decoding convergence, codes constructed following these frameworks can approach the Shannon limit closely, assuming the block length is infinite, the code structure is random and the number of decoding iterations is unbounded. However, from the perspective of practical implementation, the random structure usually leads to prohibitive encoding/decoding complexity and memory requirements. For this reason, structured LDPC codes that can achieve a better tradeoff between power efficiency and implementation simplicity have become a more appealing option for system designers. For instance, the error control codes adopted by ETSI Second Generation Digital Video Broadcasting Standard for Satellite Channels (DVB-S2), IEEE 802.11n and IEEE 802.1 le standards all belong to the category of structured LDPC codes.
On the other hand, the DVB-S2 LDPC codes family, which were originally designed for forward error control in satellite communications, have been reused by DVB-T2 (Second Generation DVB Standard for Terrestrial Channels), and are strongly recommended for DVB-C2 (Second Generation DVB Standard for Cable Channels). In addition to the consideration for system compatibility, the main reason behind the reuse of DVB-S2 codes can be attributed to their universal superior performance under various channel conditions. However, to meet the demand by cable operators for higher spectral efficiency and flexible throughputs, a technical challenge for reusing the DVB-S2 codes in DVB-C2 lies in the mapping of the given codes to constellations of very high order, which range from 256-QAM to 4096-QAM.